Thesis Deciphering the Late Jurassic Paleoenvironment Through Re-Os Isotope Geochemistry of the Agardhfjellet Formation, Svalbar
Total Page：16
File Type：pdf, Size：1020Kb
THESIS DECIPHERING THE LATE JURASSIC PALEOENVIRONMENT THROUGH RE-OS ISOTOPE GEOCHEMISTRY OF THE AGARDHFJELLET FORMATION, SVALBARD Submitted by Marisa Leigh Connors Department of Geosciences In partial fulfillment of the requirements For the Degree of Masters of Science Colorado State University Fort Collins, Colorado Summer 2017 Master’s Committee: Advisor: Judith Hannah Co-Advisor: Holly Stein Thomas Borch Øyvind Hammer i Copyright by Marisa Leigh Connors 2017 All Rights Reserved ii ABSTRACT DECIPHERING THE LATE JURASSIC PALEOENVIRONMENT THROUGH RE-OS ISOTOPE GEOCHEMISTRY OF THE AGARDHFJELLET FORMATION, SVALBARD Accurate interpretations of environmental change throughout Earth’s history rely on robust correlations of sedimentary systems. The Late Jurassic has been difficult to correlate regionally and globally due to sparse radiometric ages and lack of cosmopolitan fossils. The rhenium-osmium (Re-Os) geochronometer provides an excellent platform to approach this problem. Re-Os geochemistry provides a way to directly date organic-rich shales which can then be used to: 1) place numerical ages on boreal fossil zones and Geologic Time Scale stage boundaries, 2) correlate with regional or global units, and 3) enhance the understanding of oceanic anoxic events (OAEs) and climactic shifts when paired with additional chemical and lithological data. The Agardhfjellet Formation of Svalbard, Norway has multiple intervals of organic-rich mudrocks which are ideal for Re-Os geochemistry. Presented here are Re-Os ages which confirm placement of the Agardhfjellet Formation within the Late Jurassic to Early Cretaceous (157.9 ± 2.9 Ma to 141 ± 20 Ma). We provide an age immediately above the Kimmeridgian-Oxfordian boundary, within the Amoebocera subkitchini zone, at 153.2 ± 5.0 Ma in agreement with previous work. Furthermore we present evidence that: (1) the Agardhfjellet Formation was deposited in fluctuating anoxia conditions; (2) increasing initial 187Os/188Os (0.401±0.007 to 13 0.577±0.054) coupled with a decrease in δ Corg (-25.26‰ to -29.63‰) through the Late Jurassic ii signifies a changing climate represented by an increase continental weathering with a warming climate and/or an increase in continental freeboard. iii ACKNOWLEDGEMENTS First, I would like to thank my two advisors, Dr. Judith Hannah and Dr. Holly Stein of the Geosciences department at Colorado State University. Their support, patience, and guidance over the past three years have been greatly appreciated. I would like to thank my external committee members Dr. Thomas Borch and Dr. Øyvind Hammer for their understanding in the condensed time-line for defense. I would also like to thank Dr. Holly Stein and Dr. Øyvind Hammer for braving polar bears and the Arctic in January to select the cores for my thesis, in addition to providing the CT scan work through University of Oslo. I would like to thank our research group in the AIRIE (Applied Isotope Research for Industry and Environment) Program for support and help, especially Rich Markey who provided much of the training and background needed to preform Re-Os analyses and complete my thesis. I would also like to thank Eni Norge, Lundin Petroleum, and Aker BP (formerly Det Norske) for their generous support through the CHRONOS project, and the AAPG Foundation Grants-In-Aid program for their funding support through the Ike Crumbly Minorities in Energy Named Grant – 2015. I would like to thank my family for always encouraging me to reach for the stars (or Earth!), especially my sister Liana for sharing in the scientific graduate school experience. Lastly I would like to thank my husband, Gregory Connors for his unending support throughout this process. I could not have done it without you. iv TABLE OF CONTENTS ABSTRACT .................................................................................................................................... ii ACKNOWLEDGEMENTS ........................................................................................................... iv LIST OF TABLES ......................................................................................................................... vi LIST OF FIGURES ...................................................................................................................... vii 1. INTRODUCTION .......................................................................................................................1 2. GEOLOGIC SETTING ...............................................................................................................4 3. METHODS AND MATERIALS .................................................................................................9 Lithological Analyses ..........................................................................................................9 Re-Os Analyses ..................................................................................................................11 External Chemical Analyses ..............................................................................................12 4. RESULTS ..................................................................................................................................14 Core and CT Scan Description ..........................................................................................14 Thin-Section Petrography ..................................................................................................17 Re-Os Ages and Osi Results ..............................................................................................20 Rock-Eval Results ..............................................................................................................24 Stable Isotope Results ........................................................................................................27 Major and Trace Element Results ......................................................................................28 5. DISCUSSION ............................................................................................................................32 Ages ...................................................................................................................................32 Local stratigraphy ..................................................................................................32 Bioturbation and Re-Os ages .................................................................................33 Regional comparisons ............................................................................................33 Implications for the Late Jurassic timescale ..........................................................35 Anoxia and Restriction ......................................................................................................35 Mid-Late Jurassic Climate Shift .......................................................................................36 6. CONCLUSIONS........................................................................................................................39 LIST OF REFERENCES ...............................................................................................................40 APPENDIX A ................................................................................................................................46 APPENDIX B ................................................................................................................................52 APPENDIX C: RE-OS ISOCHRONS ...........................................................................................53 v LIST OF TABLES TABLE 1 –CORE SAMPLE INFORMATION WITH BIOSTRATIGRAPHIC CONSTRAINTS ................................................................................................................10 TABLE 2 –UNREFINED AND REFINED RE-OS AGES WITH BIOSTRATICRAPHIC AGES .................................................................................................................................21 TABLE 3 –GEOCHEMICAL DATA ...........................................................................................46 TABLE 4 –RE-OS TOTAL ANALYTICAL BLANK DATA .....................................................52 vi LIST OF FIGURES FIGURE 1 –REGIONAL AND SAMPLE LOCATION MAPS .....................................................4 FIGURE 2 –PALEOGEOGRAPHY AND STRATIGRAPHIC RELATIONSHIPS .....................5 FIGURE 3 –COMPOSITE SECTION OF BIOSTRATIGRAPHY AND LITHOGRAPHY .........7 FIGURE 4 –COMPOSITE IMAGE OF SELECTED LITHOGRAPHIC FEATURES ...............15 FIGURE 5 –PHOTOMICROGRAPHS AND INSET SEM IMAGES .........................................18 FIGURE 6 –TWO EXAMPLES OF AGE REFINEMENT TECHNIQUES ................................22 FIGURE 7 –REFINED RE-OS AGES WITH OSI ......................................................................24 FIGURE 8 –GEOCHEMICAL PLOTS WITH DEPTH ...............................................................25 FIGURE 9 –ROCK-EVAL DATA PLOTS ...................................................................................26 FIGURE 10 –REDOX PROXY PLOTS........................................................................................29 FIGURE 11 –COMPOSITE OSI FROM MIDDLE JURASSIC TO EARLY CRETACEOUS ...37 vii 1. INTRODUCTION Accurate interpretations of Earth’s paleoenvironmental and biological changes throughout time rely heavily on robust correlations of sedimentary systems. In some segments of Earth time, i.e. Late Jurassic, correlation issues arise from lack of